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# Electricity and Magnetism (Edexcel) watch

1. Need help with 2 Qs:

1) A horizontal metal rod of 1.5m long is aligned in and East-West direction and dropped from rest from the top of a high building. Calculate the e.m.f. induced across the falling rod 2.5s after release. The horizontal component of Earths magnetic field is 2x10^5 T.

Surely you need the area perpendicular to field lines, which would be the area of a circle. so you need the radius of the rod?

2) Explain why positive paint drops are attracted to the metal object. This is about an electrostatic paint sprayer, and the metal object is earthed.

Many thanks. This is URGENT by the way!!!!!
2. (Original post by darkenergy)
Need help with 2 Qs:

1) A horizontal metal rod of 1.5m long is aligned in and East-West direction and dropped from rest from the top of a high building. Calculate the e.m.f. induced across the falling rod 2.5s after release. The horizontal component of Earths magnetic field is 2x10^5 T.

Surely you need the area perpendicular to field lines, which would be the area of a circle. so you need the radius of the rod?
Isn't the area perpendicular to the field lines just the length of the rod (assuming its thickness to be negligible)?
3. the rod is horizontal, and the area perpendicular to field lines must be the cross-sectional area?
4. hi, you need to calculate the velocity of the falling rod, i.e. v=u+at

This will give you v, as you know it was stationary (u=0), a = 9.8ms-2, therefore v = 9.8 x 2.5 = 24.5ms-1

You know the length, therefore, the bar cuts 1.5 x 24.5 m in the time of 2.5 seconds.

This gives you the area of magnetic field cut, a square if you will, and then you times this by the value of B, i.e. 2x10^5, and you get your answer.

The second one, well, there is an electric field around the earthed metal object to be painted, or you can think of the charged paint particles as having there own electric fields. As such, there are electric field strengths, i.e. which show that the further away the charged paint drop is from the earthed metal object, the weaker its electric field strength (inverse square law). As the paint drop approaches the metal object, the electric field strength increases and as such the attractive force between the two increases. When the positive charge attaches to the metal object, the electrostatic forces of attraction adhere it to the surface of the metal object.

I think thats the explanation for the metal/paint spray but not 100%.
5. Thanks mate

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